It is conventional practice to employ one or more diode lasers to optically pump the main laser, e.g. an Nd:YAG laser. This pumping takes the form of an optical pulse of a wavelength that falls within a strong absorption portion of the spectrum of the material in the main laser that is to be pumped preparatory to each firing of such laser. In order for the pumping effect to be optimum this wavelength alignment between the optical pumping pulse and the absorption spectrum of the material in the main laser must be maintained within a comparatively narrow tolerance. However, the wavelength of the optical pumping pulse is relatively sensitive to the temperature of the diode junction in the pumping laser. The formation of a series of optical pulses in the diode laser generates a significant amount of heat in the diode junction. Typically only about 40% of the energy of each firing of the diode laser appears as an optical output, the remainder becoming heat in the diode junction. The diode laser is provided with a cooling system, so that it is possible by suitable adjustment of this system to cause the diode junction to achieve and retain a substantially constant equilibrium temperature, and hence to produce a constant wavelength of its optical output.
This equilibrium is, however, only maintained if the common pulse repetition rate of the two lasers remains unchanged so that the amount of heat generated per second in the diode junction remains unchanged. This consideration has represented a major disadvantage of prior systems of this type. The user must either use the main laser at a constant repetition rate, in order to keep the repetition rate of the diode laser also constant, or, if he wants to vary this rate, he must readjust the cooling system of the diode laser to regain the required equilibrium temperature of the diode junction. Such a readjustment requires skill and time.